1. Field of the Invention
The present invention relates to an articulated structure of a multiple-axis robot, as well as a multiple-axis robot comprising such a structure.
Multiple-axis robots are used to manipulate pieces quickly, along trajectories close to a plane. This is for example the case of picking up a piece moving on a conveyor belt and placing it on a flat packaging member.
2. Brief Description of the Related Art
Robots of the SCARA (Selective Compliance Assembly Robot Arm) type are multiple-axis robots suitable for working on a horizontal plane. Traditionally, a SCARA robot comprises a fixed frame, which may for example be placed on a table. An arm is also provided that is movable relative to the frame, at least in rotation, in particular around a vertical axis. Lastly, this robot is equipped with a member, such as a flange, that makes it possible to receive a tool, which is for example a gripping claw or a grinding tool. The member receiving the tool is mounted on a forearm, which in turn is rotatably mounted relative to the arm. Furthermore, the member for receiving the tool is rotatable relative to that forearm.
These robots should, however, also be able to move in a vertical plane. This vertical movement is generally performed by a linear bearing system, for example described in FR-A-2628170. The member for receiving the tool is then mounted on a shaft whereof the outer surface includes helical grooves and longitudinal grooves. Two sockets are mounted on the grooved shaft by means of balls housed in the grooves of the shaft, the inner surface of the first socket or “nut-socket” being provided with helical grooves and the second socket or “slide-socket” being provided with longitudinal grooves. The sockets are rotated separately and allow the grooved shaft, and therefore also the member receiving the tool, to move axially, to rotate, or to perform a combined rotational and translational movement.
In the event electricity to the robot is lost, the tool, or more generally the load of the robot, is driven downward by gravity in a combined translational and rotational movement, which is not in compliance with security requirements, since the robot and the load of the robot risk being damaged or injuring an operator.
To ensure vertical immobilization of the load in the event power is lost, it is known to equip the motors that drive the sockets with brakes which, when electricity is lost, immobilize the output shaft of the motors. These brakes are generally heavy, which is detrimental to the inertia of the moving parts of the robot, when the motors are fixed to those moving parts. The solution is therefore unfavorable for applications where the robot performs movements at a high speed.
Alternatively, EP-A-1852225 proposes securing the two sockets in rotation to brake the grooved shaft. The two sockets are arranged in the frame of the robot. To brake the grooved shaft, an electromagnet and a slitted ring slidingly mounted on the grooved shaft are provided below the two sockets, in the lower portion of the grooved shaft. The slitted ring is engaged, on either side and laterally, in a two-part bearing formed by two rings, one of which is stationary and the other of which is movable. The stationary ring is secured to the lower socket, while the moving ring is provided to be pushed by a spring against the stationary ring, such that the slitted ring is stuck between the two rings of the bearing. In the operating configuration of the robot, in which it is not necessary to brake the grooved shaft, the electromagnet attracts the moving ring toward it, against the return force exerted by the spring, thereby freeing the slitted ring, which does not immobilize the sockets. In the braking configuration, the electromagnet is no longer powered and the spring pushes the moving ring of the bearing back against the stationary ring, which jams the slitted ring and immobilizes the sockets to brake the load. In the operating configuration, the slitted ring rubs on the grooved shaft. The slitted ring thus has an undesirable positioning indetermination and wear risks, which is not desirable, in particular for operation at a very fast pace.